The present invention relates generally to vehicle transmissions and, in particular, to a vehicle transmission such as a four wheel drive torque transfer case which includes a friction clutch for placing the transfer case in a four wheel drive mode.
Four wheel drive (4 WD) systems for vehicles are becoming increasingly popular, due to the enhanced traction capabilities that such systems can provide in inclement weather or off highway conditions over conventional 2 WD systems. Such 4 WD systems generally include a torque transfer case which can either be integral with or separate from the main transmission. The torque transfer case can include an input shaft connected to the output of the main transmission, a rear output shaft connected to drive the vehicle rear wheels, a front output shaft connected to drive the vehicle front wheels, and means for drivingly connecting the input shaft to the front and rear output shafts.
In one type of 4 WD system known as a "part time" system, when in a 4 WD mode, there is a direct drive connection between the front and rear output shafts of the transfer case. This direct drive connection does not permit the transfer case to accommodate different front and rear wheel speeds which can occur when turning the vehicle. Accordingly, the 4 WD mode is generally used only on a "part time" basis, such as when lower friction road surface conditions, e.g., wet or snow covered pavement, may require increased traction capabilities. When turning the vehicle under these conditions, the lower friction road surface allows a limited amount of slip between the slightly faster spinning front wheels and the road surface. Otherwise, turning such a vehicle on a higher friction surface, such as dry pavement, results in what is known as "front wheel hop".
Some vehicles are provided with a "full time" 4 WD system which is permanently engaged in all driving conditions. In this type of system, which has become known as an "all wheel drive" (AWD) system, the transfer case is generally provided with an interaxle differential for dividing torque between the vehicle front and rear differentials. The interaxle differential accommodates different front and rear wheel speeds when turning the vehicle, and thus enables the system to remain permanently engaged in all operating conditions. Also, in order to prevent excessive slipping between the front and rear wheels, and transfer cases typically include a selectively engageable clutch means which is operative to lock the interaxle differential upon sensing a predetermined slippage between the front and rear output shafts of the transfer case.